The present invention relates to a reaction heat sensor including a control and analysis circuit device for measuring reaction heat.
A control and analysis circuit device for measuring reaction heat is known, comprising at least two temperature dependent resistors through which current flows and which form a voltage divider and an analyzer circuit means connected to the voltage divider. One of the resistors is coated with a catalytically active layer and a catalytically inactive resistor is provided as a reference resistor. Both resistors are exposed to a medium containing reactive components which react with the resistor coated with the catalytically active layer to generate a heat of reaction to be measured.
For gas analysis, especially for determination of a content of a combustible gas, a reaction heat sensor can be used in oxygen-containing gases. The reaction heat sensor contains, as mentioned above, two temperature-dependent resistors through which current flows and which are connected in series with each other. A first resistor is provided with a catalytically active surface and a second reference resistor is provided without a catalytically active surface. The resistor provided with the catalytically active surface acts as a measuring resistor.
In case reactive components are present in the measured gas containing an oxygen excess, e.g. CO or other oxidizable gases or reducible gases, such as NOX, a reaction heat is produced by reaction on the catalytically active surface of the measuring resistor, which increases the temperature of the measuring resistor. Thus the value of the resistance of the measuring resistor changes. This change is analyzed with a following analysis device and used for determination of the content of oxidizable gases.
Reaction heat sensors are used, for example in order to determine the composition of exhaust gases, such as carbon monoxide, nitrogen oxide and unburned or partially burned hydrocarbons, in an internal combustion engine, so that the internal combustion engine can be operated so that the emission of hazardous pollutants is kept as small as possible.
This type of reaction heat sensor, is described in DE-OS 40 20 385. One embodiment of this known sensor has two temperature sensing elements, which are both heated by a heating resistor. One of the temperature sensing elements experiences an additional heating because of the exothermic reaction occurring on its surface, while the other temperature sensing element is arranged and/or protected so that no exothermic reaction results.
This known heat content sensor has the disadvantage however that no temperature control is performed, which guarantees that the temperature of the reference temperature sensing element remains constant. When the temperature of the gas being analyzed changes during measurement, inaccuracies can occur.
Another sensor for determining the gas concentration in a gas mixture by measuring reaction heat during oxidation of combustible gas is described in DE-OS 38 44 023. In this sensor thin film resistances, which act as measuring and heating resistors, are used. They are covered with a protective layer so that they do not come into contact with an oxidizable gas.
A catalytically active layer, on which oxidation occurs and additional heating occurs, is applied to the region heatable by the thin wire resistor. The gas concentration to be measured is determined from this additional heating.